In recent years, the alimentary tract has attracted attention not only for absorbing nutrients during meals but also as an internal secretory organ producing gastrointestinal hormones. In addition to ghrelin produced by the stomach, Gastric inhibitory polypeptide: GIP or Glucagon-like peptide-1: GLP-1 secreted from the small intestine have been cloned. Ghrelin has a food intake stimulatory effect and is related to energy metabolism. GLP-1 and GIP are termed incretins and act on pancreatic β-cells in response to food load to induce insulin secretion. GIP is also expressed in adipose tissue and the fact that a GIP-receptor knockout mouse does not experience an increase in body weight even when given high fat foods suggests a connection with obesity.
Thus gastrointestinal hormones are related to energy metabolism and food intake behavior and elucidation of their function is assisting in the development of methods of treating various diseases primarily diabetes. However an overall description of gastrointestinal hormones is not yet clear and many facets remain elusive.